This application is based on Patent Application No. 2000-145033 filed in Japan, the content of which is hereby incorporated by reference.
The present invention relates to a zoom lens, for example, a zoom lens suitable as a projection optical system used in a projection device (liquid crystal projector for projecting an image on a screen such as a liquid crystal panel and the like).
Liquid crystal projectors provided with a liquid crystal panel having a micro lens array to brighten the projection image have recently become very popular. Conventional zoom lenses often used in liquid crystal projectors have an F-number of approximately F2.5, but when a liquid crystal panel having a micro lens array is used, a brighter (small F-number) zoom lens is required.
It is difficult to reduce the F-number while satisfying optical performance requirements in a projection zoom lens. For example, Japanese Laid-Open Publication No. 10-268193 discloses a zoom lens of the negative-positive-positive-negative-positive type, wherein aberration correction is difficult when the zoom lens is brighter at the wide angle end with an F-number of approximately 1.5.
An object of the present invention is to provide an improved zoom lens.
Another object of the present invention is to provide a zoom lens, which is bright and provides high performance, for use in a projector.
These objects are attained by a zoom lens system comprising, sequentially from the enlargement side thereof: a first lens unit having negative optical power, a second lens unit having positive optical power, a third lens unit having positive optical power, a fourth lens unit having negative optical power, and a fifth lens unit having positive optical power, wherein the first and fifth lens units are stationary units, and during zooming from a longest focal length condition to a shortest focal length position, the second lens unit and the third lens unit move in the direction from the enlargement side toward the reduction side and the fourth lens unit moves in the direction from the reduction side toward the enlargement side, and the following conditions (1)xcx9c(4) are satisfied.
xe2x88x922.0 less than f1/fW less than xe2x88x921.1xe2x80x83xe2x80x83(1) 
1.3 less than f2/fW less than 2.7xe2x80x83xe2x80x83(2) 
3.5 less than f3/fW less than 20xe2x80x83xe2x80x83(3) 
0.6 less than f5/fW less than 1.2xe2x80x83xe2x80x83(4) 
Where fW represents the focal length of the total system at the shortest focal length position, f1 represents the focal length of the first lens unit, f2 represents the focal length of the second lens unit, f3 represents the focal length of the third lens unit, and f5 represents the focal length of the fifth lens unit.
In another aspect of the invention, a projection system comprises a light source, at least one liquid crystal panel, an optical integrating system for focusing a light from the light source on to the liquid crystal panel, and a projection optical system. The projection optical system comprises, sequentially from an enlargement side thereof to a reduction side thereof, a first lens unit having negative optical power, a second lens unit having positive optical power, a third lens unit having positive optical power, a fourth lens unit having negative optical power, and a fifth lens unit having positive optical power, wherein the first and fifth lens units are stationary units, and during zooming from a longest focal length condition to a shortest focal length condition, the second lens unit and the third lens unit move in the direction from the enlargement side to the reduction side and the fourth lens unit moves in the direction from the reduction side to the enlargement side, and at least one of the following conditions (1)xcx9c(4) are satisfied.
xe2x88x922.0 less than f1/fW less than xe2x88x921.1xe2x80x83xe2x80x83(1) 
1.3 less than f2/fW less than 2.7xe2x80x83xe2x80x83(2) 
3.5 less than f3/fW less than 20xe2x80x83xe2x80x83(3) 
0.6 less than f5/fW less than 1.2xe2x80x83xe2x80x83(4) 
Where fW represents the focal length of the total system at the shortest focal length position, f1 represents the focal length of the first lens unit, f2 represents the focal length of the second lens unit, f3 represents the focal length of the third lens unit, and f5 represents the focal length of the fifth lens unit.